There is a continuing need in the printed circuit, graphic arts, and related industries to transfer images photographically from original (positive or negative) photomasks to one or both sides of a light sensitive sheet element in a repetitive manner. In many important cases, these images must be aligned or registered precisely to locations previously established on the element. In the case where images are transferred to both sides of the element, it is often further required that the front and rear images also be registered precisely to each other. In all cases, the photographic operation is carried out in a manner which maintains, to the greatest precision possible, both the definition and the relative locations of the features of the images, while minimizing the transfer of undesired or spurious features.
Printed circuits are often prepared by a repetitive image transfer process using dry film photoresists. The equipment used to practice the photoresist process has consisted in general of discrete pieces such as a cleaner or scrubber for cleaning the copper-clad substrate boards, an oven for preheating the boards, a roll laminator applying heat to the film and board as it laminates them together, an actinic radiation exposure station, and solvent wash-out apparatus for developing a resist image on the copper substrate. Such processes and equipment are fully described in Coombs, "Printed Circuits Handbook", McGraw-Hill Second Edition (1979) and in DeForest, "Photoresist Materials and Processes", McGraw-Hill (1975).
Typically, manual transfer and positioning of the substrate board occurs between each piece of equipment which increases expense and lessens reproducibility, leading to yield loss in the process.
Various attempts have been made to automate the photoresist process but automation has been adopted to only a limited extent. Recently, however, board trimming has been successfully incorporated into an integrated high productivity cleaning and laminating system as disclosed in U.S. Pat. No. 4,293,635 and European Patent Application No. 81104013.8, U.S. Ser. No. 153,636 and U.S. Ser. No. 153,637, both filed May 27, 1980. This automation results in substantial cost savings to the user. Nevertheless, many of the subsequent steps remain highly labor intensive and prone to human error. This is particularly true of the registration and exposure steps.
Contact printing is virtually the universal method of exposure used today in printed circuit photofabrication despite certain known shortcomings. Although low in equipment costs, simple to use, and capable of excellent line definition, contact printing is labor intensive and slow (because of long vacuum draw-down times). It also is subject to losses due to damaged or dirty photomasks resulting from repeated use. This, in turn, requires frequent and expensive touch up and replacement of photomasks to avoid yield penalties. Much time is also lost in the constant and tedious process of inspecting photomasks for defects between exposure. In addition, variations in frame temperature and ambient humidity affect corner-to-corner registration, especially for large boards, unless expensive, fragile, glass photomasks are used.
Alternative exposure methods such as gap printing, projection printing and laser scanning each offer some significant advantages over contact printing. However, in the current state of development, all have serious limitations for high productivity applications and are intrinsically much higher in equipment cost.